1. (Field of the Invention)
The present invention relates to a holographic interferometer for precisely measuring the surface configurations of optical elements such as lenses and mirrors, in particular, those of various aspherical optical devices.
2. (Description of the Prior Art)
Various proposals have heretofore been made with respect to a method of precisely measuring the surface configurations of aspherical optical elements, and, in particular, holographic interferometers are well known to those skilled in the art. A typical holographic interferometer utilizes a hologram standard including a hologram pattern formed by the interference between the wave front of a reference beam and the wave front of a beam reflected or transmitted by an aspherical reference surface, or a hologram standard including a so-called "computer hologram". A computer hologram is commonly made by an electron beam drawing method or the like, after obtaining a hologram pattern from the optical design value of an aspherical reference surface through an electronic computer. The beam reflected or transmitted by an aspherical optical element being measured is diffracted by one of these types of hologram standard, and the diffracted beam is made to interfere with a reference beam, thereby obtaining interference fringes. Finally, based on the physical number and the shapes of the thus-obtained interference fringes, precise measurement is made of the error, from the aspherical reference surface of the aspherical optical element being measured.
These holographic interferometers are typically classified into the following types: Twyman-Green type; Mach-Zehnder type; and Fizeau type. A Twyman-Green interferometer is generally arranged such that light rays supplied from a light source (a laser) are split into two light beams by a beam splitter, one of the beams being used as a reference beam while the other is made to pass through the optical element being tested and a hologram standard, so as to obtain light in diffraction. The diffracted beam light is made to interfere with the reference beam. A Mach-Zehnder interferometer commonly has a construction wherein light rays supplied from a light source (a laser) are split into two light beams by a beam splitter, one of the beams being converted into a reference beam by diffraction by a hologram standard while the other is shone onto the optical element being tested, thereby forming an object light beam, and both beams are made to interfere with each other.
A Fizeau interferometer has the construction shown in FIG. 18. As shown, a light beam emanating from a light source (laser) LS is collimated by a collimator lens C and is then reflected from a beam splitter BS which consists of an inclined half mirror disposed between a focusing lens L.sub.1 and a divergent lens L.sub.2. The reflected beam is then made to be incident upon the divergent lens L.sub.2 as an incident light beam l.sub.1. After the incident beam l.sub.1 has been diverged by the divergent lens L.sub.2, the diverged beam is made to be incident upon a spherical reference surface R. The incident beam is partially reflected by the surface R, returns along the same optical path as that of the incident light beam l.sub.1, and passes through the beam splitter BS, a hologram standard H and the focusing lens L.sub.1. Finally, the light beam passes through the opening of a spacial filter SF in the form of a zero-order reference beam.
In the meantime, the incident light beam transmitted through the spherical reference surface R is refleted by an optical element T being tested (or aspherical concave mirror), to obtain an object light beam. The object light beam is then made to travel in the reverse direction and is transmitted through the beam splitter BS. The component of the transmitted light beam which is not diffracted by the hologram standard H, that is, the zero-order light, is cut off by the spacial filter SF. On the other hand, the component of the transmitted light beam which is diffracted by the hologram standard H, for example, the first-order diffracted light beam, is passed through an opening in the spacial filter SF and forms interference fringes on an interference screen or photographic film as it is combined with the zero-order reference beams thereon.